a) Field of the Invention
The present invention relates to a zoom lens system and more specifically a zoom image pickup optical system which in compact, forms a high quality image, can be manufactured at a low cost and is to be used with a camera using electronic image pickup means such as a cam coder, digital camera or the like.
b) Description of the Prior Art
In this field, a zoom optical system disclosed by Japanese Patent Kokai Publication No. Hei 6-94997 is known as a conventional example of zoom optical system which has a high vari-focal ratio on the order of 10, is compact and can be manufactured at a low cost. The zoom optical system mentioned as this conventional example consists, in order from the object side, of a first lens unit which has positive refractive power and is kept stationary during a magnification change, a second lens unit which has negative refractive power and is moved from the object side on the image side to change a magnification from a wide position to a tele position, a third lens unit which has positive refractive power and is moved from the image side on the object side to change the magnification from the wide position to the tele position, and a fourth lens unit which has positive refractive power and is movable during the magnification change. That is, this zoom optical system has a configuration which shares a vari-focal function between the second lens unit and the third lens unit, and corrects a variation of an image surface caused due to the magnification change by moving the fourth lens unit, thereby allowing the zoom optical system to be compact.
Known as another conventional examples are optical system disclosed by Japanese Patents Kokai Publication No. Hei 5-72474 and No. Hei 8-271787 and No. Hei 4-43311 each of which consists of as first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power and a fourth lens unit having positive refractive power, the third lens unit being kept stationary during a magnification change and the other lens units being movable.
Furthermore, zoom optical systems disclosed by Japanese Patents Kokai Publication No. Hei 8-94931. No. Hei 7-199069 and No. Hei 8-179206 are known as other conventional examples of optical system wherein a fourth lens unit is kept stationary and other lens units are movable during a magnification change.
Furthermore, there is a conventional example of zoom optical system disclosed by Japanese Patent Kokai Publication No. Sho 59-13212 which has a vari-focal ratio on the order of 5, and consists of first lens unit, a second lens unit and a third lens unit which are movable for a magnification change and a fourth lens unit which is kept stationary during the magnification change and has a contrived composition.
In recent years where electronic an image pickup device has a large number of picture elements, and only compactness and a low manufacturing cost but also a capability to form a high quality image is demanded for a photographic optical system.
In order to obtain a high quality image, it is required for an optical system not only to enhance contrast and resolution but also to correct chromatic aberration favorably over an entire object distance range. Furthermore, it is required for an optical system to have an F value providing pretty brightness while reserving a required amount of marginal rays.
It cannot be said that the above-mentioned conventional examples can provide images of qualities favorable sufficiently for an electronic image pickup device which has more than 2 million picture elements or an electronic image pickup device on which picture elements are arranged at a pitch narrower than 3.8 xcexcm. Furthermore, it cannot be said that the above mentioned conventional examples are compact and can be manufactured at low costs or high in productivities.
An object of the present invention is to provide a zoom image pickup optical system comprising, in order from the object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power and a fourth lens unit having positive refractive power, wherein at least the second lens unit and the third lens unit are moved along an optical axis to change a magnification, wherein the third lens unit has at least two lens components, wherein the fourth lens unit has at least two lens components, and wherein R3r1 has a sign different from that of R4f1, R3r2 has a sign different from that of R4f2, R3r3 has a sign different from that of R4f3 and R3r4 has a sign different from that of R4f4 when a radius of curvature on an image side surface of a most image side lens component of the third lens unit and a radius of curvature on an object side surface of the third lens unit are represented by R3r1 and R3r2 respectively, a radius of curvature on an image side surface and an object side surface of a lens component which is adjacent to the most image side lens component of the third lens unit with an airspace interposed are designated by R3r3 and R3r4 respectively, a radius of curvature on an object side surface and a radius of curvature on an image side surface of a most object side lens component of the fourth lens unit are denoted by R4f1 and R4f2 respectively, and radius of curvature on an object side surface and a radius of curvature on an image side surface of an image side lens component which is adjacent to a most object side lens component with an airspace interposed are represented by R4f3 and R3r4 respectively.
Another object of the present invention is to provide a zoom optical system comprising, in order from the object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power and a fourth lens unit having positive refractive power, wherein at least the second lens unit and the fourth lens unit are moved along an optical axis to change a magnification, wherein the third lens unit consists, in order from the object side, a front subunit and a rear subunit, wherein the fourth lens unit consists, in order from the object side, a front subunit and a rear subunit, wherein the rear subunit of the third lens unit and the front subunit of the fourth ions unit and lens components which have concave surface opposed to each other, wherein either of the rear subunit of the third lens unit and the front subunit of the fourth lens unit is a meniscus cemented lens component, a cemented lens component having negative refractive power or a single meniscus lens element having negative refractive power, wherein a radius of curvature R5r1 on an image side surface of the lens component used as the rear subunit of the third lens unit has a sign different from that of a radius of curvature R4r1 on an object side surface of the front subunit of the fourth lens unit, and wherein a radius of curvature R3r2 on an object side surface of the lens component used as the rear subunit of the third lens unit has a sign different from that of a radius of curvature on an image side surface of the front subunit of the fourth lens unit.
Another object of the present invention is to provide a zoom image pickup optical system comprising, in order from the object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power and a fourth lens unit having positive refractive power, wherein at least the second lens unit and the fourth lens unit are moved along an optical axis to change a magnification, wherein the third lens unit has at least two lens components, wherein the fourth lens unit consists of four or fewer lens elements including, in order from the object side, a front subunit consisting of a lens component which has a concave surface on the object side and a rear subunit consisting of one or more lens components, and wherein the front subunit of the fourth lens unit is a meniscus cemented lens component, a cemented lens component having negative refractive power or a single negative meniscus lens element.
Still another object of the present invention is to provide a zoom image pickup optical system comprising, in order from the object side, a first lens unit having positive refractive power, a second ions unit having negative refractive power, a third lens unit having positive refractive power and a fourth lens unit having positive refractive power, wherein at least the second lens unit and the fourth lens unit are moved along an optical axis to change a magnification, and wherein the fourth lens unit consists of a lens element which has aspherical surfaces on both sides: the object side surface having an aspherical surface which changes refractive power in a negative direction as portions of the aspherical surface are farther from the optical axis toward marginal portions.
Further another object of the present invention is to provide a zoom image pickup optical system comprising, in order from the object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power and a fourth lens unit having positive refractive power, wherein at least the second lens unit and the fourth lens unit are moved along an optical axis to change a magnification, and wherein refractive indices of all lens elements which compose the optical system are 1.6 or higher.